Classifications

• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04J—MULTIPLEX COMMUNICATION
  • H04J3/00—Time-division multiplex systems
  • H04J3/02—Details
  • H04J3/06—Synchronising arrangements
  • H04J3/0635—Clock or time synchronisation in a network
  • H04J3/0638—Clock or time synchronisation among nodes; Internode synchronisation
  • H04J3/0641—Change of the master or reference, e.g. take-over or failure of the master
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04J—MULTIPLEX COMMUNICATION
  • H04J3/00—Time-division multiplex systems
  • H04J3/02—Details
  • H04J3/06—Synchronising arrangements
  • H04J3/0635—Clock or time synchronisation in a network
  • H04J3/0638—Clock or time synchronisation among nodes; Internode synchronisation
  • H04J3/0644—External master-clock

Definitions

• the present invention relates to a method of time synchronisation in communication systems. More specifically the invention relates to a method of providing a distributed method of obtaining a global time synchronisation in a communication device by utilising a beacon signal from another communication device. The invention also relates to a corresponding computer program product and communication device.
• VANET vehicular ad-hoc network
• UTC coordinated universal time
• Time synchronisation in communication systems is a task that needs to be considered carefully, especially if precise time synchronisation is required. This is especially true in high mobility ad-hoc networks, such as VANET, where no locally centralised solution is considered to be feasible for providing the required global time synchronisation in vehicular environment.
• WiMedia system described in ECMA International, High Rate Ultra Wideband PHY and MAC Standard, Standard ECMA-368, 1 st Edition, December 2005 achieves and maintains the local time synchronisation by periodically exchanging beacons among neighbouring stations in a distributed way.
• WiMedia system can only provide local synchronisation, but not global time synchronisation.
• the time synchronisation functions from GPS and Galileo systems are the preferred solutions for synchronising the local clock to the global time reference, e.g. UTC, with sufficient precision among all vehicle stations.
• the GPS system can provide 1 pulse per second (pps) time synchronisation signal with a precision of 200ns, which is sufficient for current communication systems.
• pps pulse per second
• the availability of GPS signal can not be guaranteed in all locations. For instance vehicle stations may lose the time synchronisation when they drive into a tunnel or a building, where the satellite signal can not be received anymore.
• the local clock at each station may drift from the global time reference as time goes by.
• the offset of the local clock from the global time reference depends on the elapsed time since last synchronisation and the local clock skew, which describes the frequency shift of the local clock. Therefore, a scheme is needed to maintain the global synchronisation achieved through the precise time reference in case this precise time reference is not available for some or all of the stations.
• the present invention provides a method of maintaining the global time synchronisation achieved based on the global time references from satellite positioning systems, such as GPS or Galileo, when the global time reference is no longer available for some or all of the devices.
• satellite positioning systems such as GPS or Galileo
• a computer program product comprising instructions for implementing the method according the first aspect of the invention when loaded and run on computer means of the first communication device.
• a communication device arranged for implementing the method according to a first aspect of the present invention as recited in claim 7.
• FIG.1 shows the control and service channels of IEEE 1609 along a timeline
• FIG.2 is a simplified block diagram of the communication device in accordance with an embodiment of the present invention
• - FIG.3 is a simplified block diagram of a time synchronisation block in accordance with an embodiment of the present invention
• FIG.4 shows one beacon period, comprised of beacon slots along a timeline; and - FIG.5 shows a flow chart in accordance with an embodiment of the present invention.
• FIG.1 there are shown two channel intervals of the IEEE 1609 system, namely a control channel (CCH) interval and a service channel (SCH) interval. These intervals are globally synchronised.
• CCH control channel
• SCH service channel
• CCH control channel
• every station has to stay on the control channel for the purpose of exchanging critical danger warning messages
• stations can optionally switch to other service channels for performing non- safety applications, but before the starting time of the next CCH interval every station must switch back again to the control channel.
• Beacon signals are also transmitted on the control channel.
• FIG.2 shows a simplified block diagram of a communication device 200, in this example a vehicular communication on board unit (OBU), where the teachings of the present invention can be applied.
• the device 200 contains a transmission/reception antenna (TX/RX) 201 for transmitting and receiving data.
• TX/RX transmission/reception antenna
• a single antenna can be used for both the transmission and reception. In this case the antenna is used for vehicular communication.
• the TX/RX antenna 201 is connected to a transmission block 203 and a receiver block 205.
• These communication blocks take care of the transmission and reception of packets according to certain communication protocol, e.g., IEEE 802.1 1 p, which is used to implement the medium access (MAC) layer of the IEEE 1609 standard.
• MAC medium access
• the device 200 further comprises a GPS antenna 207 and a GPS receiver 209, which is connected to the GPS antenna 207. These units are needed for receiving the GPS signal, which provides the global time reference.
• the GPS elements could equally be replaced with any other satellite positioning system elements. For instance, the system could be arranged to receive Galileo signals instead of GPS signals. It is also possible that the device 200 is arranged to receive signals from different satellite positioning systems.
• the device 200 also comprises a local clock 21 1 , which serves as a local oscillator providing the local time reference for all the blocks. Time offset and frequency skew are the main reason of system synchronisation problem. For correcting the clock skew of the local oscillator in local clock 21 1 , the device comprises a local clock skew correction block 213. Then there is also shown a time synchronisation block 215, which is the central block for implementing the synchronisation method in accordance with the present invention.
• the time synchronisation block 215 comprises a beacon generator 301 for generating the beacon frame, which carries the information of local time quality.
• the beacon generator 301 is connected to the transmission block 203 through a transmission interface 303.
• the time synchronisation block 215 further contains a beacon analyser 305 for analysing the time information from the received beacon.
• the beacon analyser is connected to the receiver block through a receiver interface 307.
• the time synchronisation block 215 also contains a local time quality estimator 309 for tracking the time quality of the local clock.
• a synchronisation controller 31 1 makes the decision on adjusting the local clock according to the received beacon or GPS time information.
• the synchronisation controller 31 1 is connected to the GPS receiver 209, local clock skew correction 213, local clock 21 1 , local time quality estimator 309, beacon analyser 305 and beacon generator 301.
• a distributed beaconing scheme is used.
• all devices of the communication system follow the globally synchronised system structure, which includes a beacon period consisting of multiple beacon slots of equal length, as shown in FIG.4.
• a beacon is always transmitted at the start time of each beacon slot and carrying the slot number it used.
• Upon receiving a beacon frame one can derive the clock difference between the sender's and its own clocks by calculating and comparing a common time reference point, e.g. the beacon period start time.
• Propagation delay also affects timing uncertainty, but in a short-range network propagation delays are small and thus they can be ignored.
• the present invention provides a method for maintaining the global time synchronisation achieved by receiving the global time references from satellite positioning systems, such as GPS or Galileo systems, when the global time reference is no longer reachable by some or all of the devices.
• satellite positioning systems such as GPS or Galileo systems
• the idea is that in addition to the time synchronisation using the satellite positioning system signal, a "local" synchronisation algorithm based on the distributed beaconing scheme is employed in this method.
• the distributed synchronisation method forms a "local" time basis among the neighbouring devices and keeps the "local” time basis as close as possible to the global time reference.
• the devices exchange the clock time quality information using a distributed beaconing scheme.
• a device 200 receiving a beacon which has a higher clock time quality than its local one, adjusts its local clock and clock time quality according to the received beacon.
• the local clock time quality is determined by the time since last synchronisation action and the local clock skew.
• a higher time quality indicates a smaller offset from the global time reference, while a lower time quality indicates a bigger time offset from the global time reference.
• each device applies the local clock skew correction depending on the received high precision global time reference.
• all the devices in the mutual communication range will synchronise to the one who has the highest clock time quality, i.e. the closest clock to the global time reference, even if some of the devices cannot receive the satellite signal directly.
• step 501 the device 200 is in a "starting up" state and determines whether it has received a GPS signal or a valid beacon signal. If no signal has been received during a certain period of time, there is a scanning time out. In this case the device 200 sets the local time quality to the worst value and transits to an unsynchronised state. The device 200 then waits until a valid timing signal has been received. If on the other hand a valid timing signal is received in step 501 , then in step 503 the device 200 adjusts its local clock to correspond to the received timing signal and the timing quality is updated. Now the device 200 operates in a synchronised state.
• the device 200 adjusts its local clock 21 1 according to the time signal from the GPS and resets the local time quality to the best value. If on the other hand the received signal is a beacon signal then the device 200 adjusts the local clock according the sender's clock and sets the time quality to the same value as indicated in the received beacon.
• step 505 it is determined whether a GPS signal is received.
• the GPS signal may be received once in a second and it provides a precision of 200 ns. If the GPS signal is received, then in step 507 the device 200 adjusts its local clock according to the timing signal from the GPS and updates its timing quality by setting the timing quality to the best possible value. The local clock skew is also corrected. Thus the device 200 remains in the synchronised state.
• step 509 the procedure continues in step 509 by determining whether a beacon is received. Also from step 507 the method continues directly in step 509. If in step 509 it is determined that no beacon signal is received, then in step 515 the local clock is again adjusted and timing quality updated. If on the other hand in step 509 it is determined that a beacon is received, then in step 51 1 the device 200 determines whether the timing quality of the received beacon is valid. In this example the beacon is received periodically. The device 200 calculates the local clock value of the beacon sender by analysing the received beacon.
• step 512 the quality of the local clock of the receiving device 200 is determined and once this is done, it is determined in step 513 whether the received timing quality is greater than the timing quality of the local clock, i.e. whether the timing accuracy of the received beacon is of higher precision than the accuracy of the local clock of the receiving device 200. If this is the case, then in step 515 the local clock of the receiving device 200 is adjusted and the timing quality is updated.
• the receiving device 200 has to continuously listen to beacons from the neighbouring devices and it also has to a transmit beacon at a chosen beacon slot in every beacon period. The local time quality is carried by the beacon signal.
• step 517 a cumulative timing quality calculation is calculated. Also if in steps 51 1 or 513 the answer is negative, the procedure continues directly in step 517.
• the device 200 For obtaining the cumulative timing quality, the device 200 has to keep track on the local timing quality at each local clock tick. The timing quality of the local clock deteriorates by the step of T t ⁇ x ClockSkew till the worst clock quality is reached unless the local clock 21 1 is adjusted.
• T t , Ck denotes the clock resolution and the ClockSkew is the frequency skew of the local oscillator.
• step 519 it is determined whether the cumulative timing quality is below a threshold value. If this is the case the procedure continues in step 501 and the device 200 moves to unsynchronised state. On the other hand if the cumulative timing quality is still larger than the threshold, then the procedure continues in step 505 and the device 200 remains in the synchronised state.
• the frequency skew of the local oscillator is a parameter set by the manufacturer of the system, and which is statistically estimated as the worst case. This means that even if the clock skew of a considered system is far lower than the worst case (i.e. the clock quality is better than estimated), the quality indicator will drop quickly, and will not represent well the quality of the local clock. Moreover, the frequency skew value can also vary with the time, because of ageing of the crystal for instance, thus the frequency skew value could become underestimated.
• this estimated frequency skew value is that it can be used to create a compensation of the local clock skew, in order to correct the deviancy of the local clock.
• the estimated frequency skew tells whether the local clock is slower or faster than the global time reference, and gives an estimation of the amount.
• the local clock is periodically compensated with help of the estimated ClockSkew, towards the minimum frequency skew regarding the global time reference.
• the global time signal is not available, the local clock is used, and its deviancy has been corrected with help of the estimated ClockSkew, simulating thus a more accurate local clock.
• VCXO voltage controlled crystal oscillator
• error estimation circuit which estimates the frequency skew value.
• the clock frequency can be adjusted with a voltage generated by a control circuit on the basis of a signal representative of the frequency skew value.
• the invention thus provides a distributed beaconing algorithm by which each device periodically sends its beacon, each beacon carrying the local time quality describing the estimated local clock shift from the global time reference.
• Each device locally estimates its clock time quality according to the time elapsed since last synchronisation and the local clock skew. Alternatively the clock time quality is calculated by multiplying the time elapsed by the local clock skew.
• the invention equally relates to a computer program product that is able to implement any of the method steps of the embodiments of the invention when loaded and run on computer means of the device 200.
• the method is implemented on each device and performs the synchronisation functionality in a distributed way.
• the invention equally relates to an integrated circuit that is arranged to perform any of the method steps in accordance with the embodiments of the invention.
• a computer program may be stored/distributed on a suitable medium supplied together with or as a part of other hardware, but may also be distributed in other forms, such as via the Internet or other wired or wireless telecommunication systems. Any reference signs in the claims should not be construed as limiting the scope of the invention.
• a method of synchronising a first communication device (200) with a second communication device in a radio communication system, where the first and second communication devices comprise local clocks (21 1 ) for obtaining local time signals comprises the following steps performed by the first communication device (200):
• beacon signal from the second communication device, the beacon signal comprising a time signal of the second communication device and the quality of the time signal;
• the method further comprises obtaining (505) a timing signal from a global reference and based on this signal, performing a synchronisation by correcting the accuracy of the local clock (21 1 ) of the first communication device (200) and updating the quality of the of local clock (21 1 ) signal of the first communication device (200).

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• Engineering & Computer Science (AREA)
• Computer Networks & Wireless Communication (AREA)
• Signal Processing (AREA)
• Synchronisation In Digital Transmission Systems (AREA)
• Mobile Radio Communication Systems (AREA)
• Electric Clocks (AREA)

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Citations (4)

• 2008
  • 2008-08-19 TW TW97131615A patent/TW200926660A/zh unknown
  • 2008-08-21 WO PCT/IB2008/053361 patent/WO2009024945A2/en active Application Filing

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